In a vehicle behavior control technique such as a vehicle stability control (VSC), various controls are performed by using a detection value of a steering angle sensor attached to a steering wheel. When a steering angle sensor that outputs a relative angle is used, a zero point of the steering angle sensor is first detected and an absolute steering angle is calculated based on the detected zero point. Therefore, it is important to accurately detect the steering angle zero point. For example, Patent Literature 1 discloses a technique for detecting the steering angle zero point on the basis of a difference between an estimate yaw rate that is estimated from the detection value of the steering angle sensor and an actual yaw rate detected by a yaw rate sensor.